A rechargeable secondary battery unit is used for a mobile notebook personal computer (PC). Although a fuel cell unit as a new electric energy source has been developed, a fuel cell which alone can supply sufficient current to a notebook PC has not been realized. At present, a fuel cell unit cannot provide sufficient current for a notebook PC when it is used alone. Accordingly, it has been proposed to use a secondary battery unit in combination with a fuel cell unit for a notebook PC.
Japanese Patent Application Publication JP 2006-73503-A laid open to the public on Nov. 11, 2004 discloses a fuel cell system. This fuel cell system includes a fuel cell stack, a fuel supply part to supply fuel to the fuel cell stack, a secondary battery as an accumulator device, a bidirectional DC/DC converter which selectively performs an operation of converting the output voltage of the secondary battery to a prescribed voltage to output it, and an operation of charging the secondary battery by using the output power of the fuel cell stack, and a mode control circuit. The mode control circuit detects the output voltage of the fuel cell stack to cause the bidirectional DC/DC converter to perform the charge operation, when the output voltage of the fuel cell stack is higher than a set value, and to cause the bidirectional DC/DC converter to perform the discharge operation, when the output voltage of the fuel cell stack is not higher than the set value. Thus, an accumulator device can be charged without putting an external load in a stopped state.